1. Field of the Invention
The invention relates generally to force-motion measurement devices, and specifically to devices for measuring the force-motion and energy of waves moving through solid (metallic) objects.
The Combat System Test Activity (hereinafter referred to as CSTA) velocity gage of the prior art uses the principle of electromagnetic induction to generate a signal. The signal amplitude is proportional to the speed of the movement of a coil of fine wire with respect to a relatively stationary magnet.
The instant invention, the Inertial Hopkinson Bar Sensor (hereinafter referred to as IHOP) gage, instead measures the actual expansion and contraction of the metal surface of the rod using a strain gage. The IHOP gage is passive because only the resistance of the strain gage changes as it stretches. An electrical current supplies the excitation. The IHOP gage gives a direct measurement of the contraction of the rod and thus of the amplitude of the deformation wave moving through the plate.
The chief advantage in the IHOP gage is its resistance to breakdown or detachment, when subjected to very large amplitude high frequency shock, such as that experienced in close proximity to the impact point of a large high speed projectile.
2. Description of the Prior Art
Devices of various types have been used for many years in attempts to measure the energy of projectiles and their effect upon armor plate. The present state of the art includes strain gages of various types as well as several types of velocity gages which use electromagnetic effects to measure the velocity of a surface. A summary of the present state-of-the-art is presented in a paper by Mr. Willard Scott Walton entitled "New Ballistic Shock Protection Requirement for Armored Combat Vehicles" contained in the 60th Shock and Vibration Symposium Vol. I (David Taylor Research Center, Bethesda, Md.).
Mr. Hopkinson reported the use of an instrumented bar to measure shock in 1919. His work has been built upon by many investigators since that time. Notable among them is Mr. R. Sill who reported upon the use of the Hopkinson bar to calibrate sensors. The Hopkinson bar concept is presently being used in many laboratories to calibrate sensors and to experiment with shock waves.
References teaching shock measurement including those measuring shock of impact upon armor plates or solid bodies are well known in the art. Exemplary of those include U.S. Pat. No. 4,379,401, issued to Anthony San Miguel which teaches a system designed to measure only the amplitude of maximum plate movement (deformation).
The plate is pushed backward at high speed by some force and the apparatus as described is akin to a series of off-on switches at different distances from the plate. The plate hits each switch sequentially as it moves backward and the last switch to be contacted gives a rough approximation of the total motion of the plate.
U.S. Pat. No. 3,872,709 issued to Victor H. Pagano, discloses a method for measuring the resistance of a plate which is being fractured by some dynamic force. The method provides no data for the shock or vibration of tile plate, no time-amplitude history of the deformation. The method can be used only once per sample. The data provided is discrete. That is, it is test dependent upon the discrete number of grooves machined in the plate. The method is useful for evaluating plate quality only if the exact energy input is known.
U.S. Pat. No. 3,525,250 issued to William M. Hurst teaches mechanical gages for measuring the approximate amplitude of shock waves in air. A thin sheet of metal is pushed into grooves of various depths. A higher pressure is required to deform the sheet metal into the deeper grooves. First, the deformable gage is incapable of measuring shock levels in armor plate, only in air. Again no signals are produced which record the amplitude-time history of a shock wave. The gages are incapable of being used to measure shock in armor plate.
Jon S. Wilson, "Pyro-In, Garbage Out", Test Engineering & Management, pp. 12-14, June/July 1989 discusses accelerometer mounts and the CSTA velocity gage.
David Frommer, et al, "Mechanical Shock Sensors (A Feasibility Study)", 59th Shock and Vibration Symposium, Sandia Report SAND88-2473, Volume III, pp. 239-249, 1988. This reference discusses the mechanical sensor concept.
While directed to measuring shock waves in armor plates, the study was directed to various sensors such as piezo and IR film sensors, magnetic sensors and copper-ball sensors. W. Scott Walton, "Dynamic Response of Armor Plate to Non-penetrating Projectile Impact", Shock and Vibration Bulletin No. 56 pp. 135-149 1986, refers to the CSTA velocity gage. The IHOP and CSTA velocity gage have the same objective but differ in techniques as will be described, hereinbelow.